We have continued investigating the mechanisms of programmed cell death (PCD) pathway triggered by the T cell receptor (TcR) in functionally mature T cells, and have defined three distinct death pathways. The first is found in T hybridomas, activated peripheral T cells, and blood T cells from HIV+ donors, and involves Fas and Fas Ligand. Activation upregulates Fas Ligand via a calpain-dependent pathway, giving rise to cell death triggered by Fas. This pathway is blocked when hybridomas are transfected by the calpain-specific inhibitor calpastatin, as well as by other protease inhibitors. It operates in normal CD4+ and CD8+ T cell blasts triggered by the TcR, but is not involved in other apoptotic death pathways in these cells. Based on blocking cell death by protease inhibitors, it also operates when T cells from HIV+ donors are activated by superantigen. In this setting blocking this TcR-induced death pathway by protease inhibitors can allow an activation response, reversing the T helper functional deficiency previously described. A second TcR-induced death response occurs more slowly and is mediated by TNF or lymphotoxin. We have found that activated T cell can die in vitro in response to these cytokines, which have previously been found to kill only tumor cells. This death process is triggered by a mixture of antibodies against both TNF receptors, and is blocked by RNA and protein synthesis inhibitors as well as the cytokines IL-2 and IL-12. A third TcR- induced death response occurs when rigorously purified resting T cells from blood, lymph node, or spleen are exposed to anti-CD3 overnight in vitro. This death has not previously been seen because it is blocked by a variety of macrophage- and T cell-derived cytokines which are normally present. This default death pathway occurs in the Fas Ligand mutant strain gld, and is not blocked by anti-Fas antibody, suggesting a new as-yet- undefined molecular pathway of death. These results argue that a survival signal is required for normal T cell activation; this third intracellular signal is required in addition to the TcR and costimulation, although it normally occurs via IL-2 produced as a consequence of the first two signals.